1. Field of the Invention
The invention relates in general to accelerometers, and more specifically to angular accelerometers for developing a signal representative of the instantaneous acceleration of a rotating shaft.
1. Description of the Prior Art
Traction elevator systems suspend the elevator car on a plurality of wire ropes which pass over a traction sheave and are connected to a counterweight. The traction sheave is usually driven by an electrical drive motor, such as an A.C. induction motor via a reduction gear, or a D.C. motor, either directly or via a reduction gear, depending upon the contract speed of the elevator.
The mechanical system of the traction elevator, which consists of all rotational and translational inertia and cable spring compliance, behaves as a resonant system with very little damping. The oscillation frequency of the mechanical system ranges between about 3 and 15 hertz, and is a function of the aforesaid parameters, as well as the load in the elevator car, and the position and speed of the elevator car. Anything that perturbs the mechanical system at its resonant frequency can cause an annoying vertical oscillation of the elevator car, referred to as jitter.
A source of the perturbance, which may be produced with either a motor generator voltage source, or a static converter voltage source, is due to the relationship between the poles and other mechanical structure of the motor, the motor speed, and drive sheave diameter. The output torque of the motor may be inherently perturbed due to its structure a predetermined number of times for each revolution of the motor, which for a predetermined motor speed and sheave diameter may translate to a perturbance frequency in the resonant frequency range of the mechanical system.
Another source of the perturbance may occur in those elevator systems in which the source of the adjustable direct current voltage is a static dual bridge converter. During bank reversal, a sudden torque change in the output shaft of the drive motor due to an abrupt armature current change may shock the mechanical system into resonance.
Jitter may be caused by electrical noise in the stabilizing signal applied to the velocity error signal in the control loop which determines the magnitude of the direct current voltage applied to the drive motor.
High speed elevator systems driven by a direct current motor, with a tachometer as the velocity feedback control element to control the velocity of the elevator car, require stabilization means in order to achieve a smooth response.
The derivature of the drive motor armature voltage, or the derivative of the counter emf developed by the armature of the drive motor, may be used as the stabilizing signal.
The ripple frequencies in these voltages must be filtered out. The filtering, however, is not without its perils, as filtering the high frequency alternating component may actually cause instability. In addition, filtering reduces the bandwidth and effectiveness of the signal.
The stabilizing signal may also be provided by taking the derivative of a tachometer voltage, which may be used to provide the velocity signal. Tachometers, however, produce electrical noise in their output signals due to slots, commutator bars, brushes, imperfections in construction, and the gearing or belting of the drive coupling. Differentiation of the tachometer signal to develop a signal proportional to acceleration accentuates the noise. This electrical noise appears as an unwanted addition to the command signal. The elevator car is capable of responding to this electrical noise in the signal at low frequencies, particularly at the system resonance frequency.
The electrical noise in a tachometer arrangement may be reduced by using a very high quality, low ripple tachometer, such as 2% maximum ripple peak-to-peak, and using a rim drive instead of a belt drive. In a rim drive, the tachometer has a roller on its drive shaft which is frictionally driven by a rotating element, such as the motor shaft, or the drive sheave, of an elevator system. Even this signal, however, must be low pass filtered, which reduces its bandwidth and effectiveness. Slippage between the roller of the tachometer and the driving element could be a problem, with U.S. Pat. No. 4,085,823, which is assigned to the same assignee as the present application, describing self-checking circuits which may be used to detect slippage.
My U.S. Pat. No. 3,749,204 discloses an acceleration transducer for providing a stabilizing signal. A disadvantage of the acceleration transducer arrangement is that a variable amount of armature voltage feedback, (a parasitic signal in this case) is an integral part of the signal. The variability is due to the change in armature inductance with field strength and to armature resistance with temperature. The stabilizing signal provides some jitter suppression, but the required amount sometimes cannot be used because the system can become unstable at some higher frequency due to the parasitic signal.
My U.S. Pat No. 4,030,570 discloses developing stabilization via two feedback signals, with one being obtained by differentiating the output of a high quality, rim driven tachometer, and another by differentiating and then integrating the signal from such a tachometer.
Considering the present state of the art, the best approach for the development of stabilization signals appears to be the use of the high quality, low ripple tachometer, notwithstanding its relatively high cost.